Low tunnel barrier insulators

ABSTRACT

Structures and methods for programmable array type logic and/or memory devices with asymmetrical low tunnel barrier intergate insulators are provided. The programmable array type logic and/or memory devices include non-volatile memory which has a first source/drain region and a second source/drain region separated by a channel region in a substrate. A floating gate opposing the channel region and is separated therefrom by a gate oxide. A control gate opposes the floating gate. The control gate is separated from the floating gate by an asymmetrical low tunnel barrier intergate insulator formed by atomic layer deposition. The asymmetrical low tunnel barrier intergate insulator includes a metal oxide insulator selected from the group consisting of Al 2 O 3 , Ta 2 O 5 , TiO 2 , ZrO 2 , Nb 2 O 5 , SrBi 2 Ta 2 O 3 , SrTiO 3 , PbTiO 3 , and PbZrO 3 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/929,986 filedAug. 30, 2004, which is a Divisional of U.S. Ser. No. 10/081,818 filedon Feb. 20, 2002, which is a Continuation-in-Part of U.S. Ser. No.09/943,134 filed on Aug. 30, 2001, now issued as U.S. Pat. No.7,042,043, which applications are herein incorporated by reference.

This application is related to the following co-pending, commonlyassigned U.S. patent applications: “DRAM Cells with Repressed MemoryMetal Oxide Tunnel Insulators,” Ser. No. 09/945,395, now issued as U.S.Pat. No. 6,754,108; “Flash Memory with Low Tunnel Barrier InterpolyInsulators,” Ser. No. 09/945,507, now issued as U.S. Pat. No. 7,068,544;“Dynamic Electrically Alterable Programmable Memory with InsulatingMetal Oxide Interpoly Insulators,” Ser. No. 09/945,498, now issued asU.S. Pat. No. 6,778,441; “Field Programmable Logic Arrays with MetalOxide and/or Low Tunnel Barrier Interpoly Insulators,” Ser. No.09/945,512, now issued as U.S. Pat. No. 7,087,954; “SRAM Cells withRepressed Floating Gate Memory, Metal Oxide Tunnel InterpolyInsulators,” Ser. No. 09/945,554, now issued as U.S. Pat. No. 6,963,103;“Programmable Memory Address and Decode Devices with Low Tunnel BarrierInterpoly Insulators,” Ser. No. 09/945,500, now issued as U.S. Pat. No.7,075,829; and “Programmable Array Logic or Memory with P-ChannelDevices and Asymmetrical Tunnel Barriers,” Ser. No. 10/028,001, nowissued as U.S. Pat. No. 7,132,711; each of which disclosure is hereinincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuits, and inparticular to programmable array type logic and/or memory devices withasymmetrical low tunnel barrier interpoly insulators.

BACKGROUND OF THE INVENTION

Flash memories have become widely accepted in a variety of applicationsranging from personal computers, to digital cameras and wireless phones.Both INTEL and AMD have separately each produced about one billionintegrated circuit chips in this technology.

The original EEPROM or EARPROM and flash memory devices described byToshiba in 1984 used the interpoly dielectric insulator for erase. (Seegenerally, F. Masuoka et al., “A new flash EEPROM cell using triplepolysilicon technology,” IEEE Int. Electron Devices Meeting, SanFrancisco, pp. 464-67, 1984; F. Masuoka et al., “256K flash EEPROM usingtriple polysilicon technology,” IEEE Solid-State Circuits Conf.,Philadelphia, pp. 168-169, 1985). Various combinations of silicon oxideand silicon nitride were tried. (See generally, S. Mori et al.,“reliable CVD inter-poly dialectics for advanced E&EEPROM,” Symp. OnVLSI Technology, Kobe, Japan, pp. 16-17, 1985). However, the rough topsurface of the polysilicon floating gate resulted in, poor qualityinterpoly oxides, sharp points, localized high electric fields,premature breakdown and reliability problems.

Widespread use of flash memories did not occur until the introduction ofthe ETOX cell by INTEL in 1988. (See generally, U.S. Pat. No. 4,780,424,“Process for fabricating electrically alterable floating gate memorydevices,” 25 Oct. 1988; B. Dipert and L. Hebert, “Flash memory goesmainstream,” IEEE Spectrum, pp. 48-51, October, 1993; R. D. Pashley andS. K. Lai, “Flash memories, the best of two worlds,” IEEE Spectrum, pp.30-33, December 1989). This extremely simple cell and device structureresulted in high densities, high yield in production and low cost. Thisenabled the widespread use and application of flash memories anywhere anon-volatile memory function is required. However, in order to enable areasonable write speed the ETOX cell uses channel hot electroninjection, the erase operation which can be slower is achieved byFowler-Nordhiem tunneling from the floating gate to the source. Thelarge barriers to electron tunneling or hot electron injection presentedby the silicon oxide-silicon interface, 3.2 eV, result in slow write anderase speeds even at very high electric fields. The combination of veryhigh electric fields and damage by hot electron collisions in the oxideresult in a number of operational problems like soft erase error,reliability problems of premature oxide breakdown and a limited numberof cycles of write and erase.

Other approaches to resolve the above described problems include; theuse of different floating gate materials, e.g. SiC, SiOC, GaN, andGaAlN, which exhibit a lower work function (see FIG. 1A), the use ofstructured surfaces which increase the localized electric fields (seeFIG. 1B), and amorphous SiC gate insulators with larger electronaffinity, χ, to increase the tunneling probability and reduce erase time(see FIG. 1C).

One example of the use of different floating gate (FIG. 1A) materials isprovided in U.S. Pat. No. 5,801,401 by L. Forbes, entitled “FLASH MEMORYWITH MICROCRYSTALLINE SILICON CARBIDE AS THE FLOATING GATE STRUCTURE.”Another example is provided in U.S. Pat. No. 5,852,306 by L. Forbes,entitled “FLASH MEMORY WITH NANOCRYSTALLINE SILICON FILM AS THE FLOATINGGATE.” Still further examples of this approach are provided in pendingapplications by L. Forbes and K. Ahn, entitled “DYNAMIC RANDOM ACCESSMEMORY OPERATION OF A FLASH MEMORY DEVICE WITH CHARGE STORAGE ON A LOWELECTRON AFFINITY GaN OR GaAlN FLOATING GATE,” Ser. No. 08/908,098, and“VARIABLE ELECTRON AFFINITY DIAMOND-LIKE COMPOUNDS FOR GATES IN SILICONCMOS MEMORIES AND IMAGING DEVICES,” Ser. No. 08/903,452.

An example of the use of the structured surface approach (FIG. 1B) isprovided in U.S. Pat. No. 5,981,350 by J. Geusic, L. Forbes, and K. Y.Ahn, entitled “DRAM CELLS WITH A STRUCTURE SURFACE USING A SELFSTRUCTURED MASK.” Another example is provided in U.S. Pat. No. 6,025,627by L. Forbes and J. Geusic, entitled “ATOMIC LAYER EXPITAXY GATEINSULATORS AND TEXTURED SURFACES FOR LOW VOLTAGE FLASH MEMORIES.”

Finally, an example of the use of amorphous SiC gate insulators (FIG.1C) is provided in U.S. patent application Ser. No. 08/903,453 by L.Forbes and K. Ahn, entitled “GATE INSULATOR FOR SILICON INTEGRATEDCIRCUIT TECHNOLOGY BY THE CARBURIZATION OF SILICON.”

Additionally, graded composition insulators to increase the tunnelingprobability and reduce erase time have been described by the sameinventors. (See, L. Forbes and J. M. Eldridge, “GRADED COMPOSITION GATEINSULATORS TO REDUCE TUNNELING BARRIERS IN FLASH MEMORY DEVICES,”application Ser. No. 09/945,514.

However, all of these approaches relate to increasing tunneling betweenthe floating gate and the substrate such as is employed in aconventional ETOX device and do not involve tunneling between thecontrol gate and floating gate through and inter-poly dielectric.

Therefore, there is a need in the art to provide improved programmablearray type logic and/or memory devices while avoiding the large barriersto electron tunneling or hot electron injection presented by the siliconoxide-silicon interface, 3.2 eV, which result in slow write and erasespeeds even at very high electric fields. There is also a need to avoidthe combination of very high electric fields and damage by hot electroncollisions in the which oxide result in a number of operational problemslike soft erase error, reliability problems of premature oxide breakdownand a limited number of cycles of write and erase. Further, when usingan interpoly dielectric insulator erase approach, the above mentionedproblems of having a rough top surface on the polysilicon floating gatewhich results in, poor quality interpoly oxides, sharp points, localizedhigh electric fields, premature breakdown and reliability problems mustbe avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate a number of previous methods for reducingtunneling barriers in Flash memory.

FIG. 2 illustrates one embodiment of a floating gate transistor, ornon-volatile memory cell, according to the teachings of the presentinvention.

FIG. 3 illustrates another embodiment of a floating gate transistor, ornon-volatile memory cell, according to the teachings of the presentinvention.

FIG. 4 is a perspective view illustrating an array of silicon pillarsformed on a substrate as used in one embodiment according to theteachings of the present invention.

FIGS. 5A-5E are cross sectional views taken along cut line 5-5 from FIG.4 illustrating a number of floating gate and control gate configurationswhich are included in the scope of the present invention.

FIGS. 6A-6D illustrate a number of address coincidence schemes can beused together with the present invention.

FIG. 7A is an energy band diagram illustrating the band structure atvacuum level with the low tunnel barrier interpoly insulator accordingto the teachings of the present invention.

FIG. 7B is an energy band diagram illustrating the band structure duringan erase operation of electrons from the floating gate to the controlgate across the low tunnel barrier interpoly insulator according to theteachings of the present invention.

FIG. 7C is a graph plotting tunneling currents versus the appliedelectric fields (reciprocal applied electric field shown) for an numberof barrier heights.

FIG. 8 illustrates a block diagram of an embodiment of an electronicsystem 801 according to the teachings of the present invention.

FIG. 9 is a table which provides relevant data on the barrier heights,energy gaps, dielectric constants and electron affinities of a widevariety of metal oxides that could be used as asymmetric tunnel barriersaccording to the teachings of the present invention.

FIG. 10 illustrates a block diagram of an embodiment of an electronicsystem according to the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. The embodiments are intended to describeaspects of the invention in sufficient detail to enable those skilled inthe art to practice the invention. Other embodiments may be utilized andchanges may be made without departing from the scope of the presentinvention. In the following description, the terms wafer and substrateare interchangeably used to refer generally to any structure on whichintegrated circuits are formed, and also to such structures duringvarious stages of integrated circuit fabrication. Both terms includedoped and undoped semiconductors, epitaxial layers of a semiconductor ona supporting semiconductor or insulating material, combinations of suchlayers, as well as other such structures that are known in the art.

The term “horizontal” as used in this application is defined as a planeparallel to the conventional plane or surface of a wafer or substrate,regardless of the orientation of the wafer or substrate. The term“vertical” refers to a direction perpendicular to the horizontal asdefined above. Prepositions, such as “on”, “side” (as in “sidewall”),“higher”, “lower”, “over” and “under” are defined with respect to theconventional plane or surface being on the top surface of the wafer orsubstrate, regardless of the orientation of the wafer or substrate. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled.

The present invention, describes the use of asymmetrical metal oxideinter-poly dielectric insulators, formed by atomic layer deposition(ALD), between the control gate and the floating gate of non-volatilememory cells. An example is shown in FIG. 2 for a planar structure, orhorizontal non-volatile memory cell. This non-volatile memory cell, asdescribed herein, can then be implemented in a number of programmablearray type logic and/or memory devices according to the teachings of thepresent invention.

According to the teachings of the present invention. The use of anasymmetrical metal oxide films, formed by atomic layer deposition (ALD),for this purpose offer a number of advantages including:

(i) Flexibility in selecting a range of smooth metal film surfaces andcompositions that can be oxidized to form tunnel barrier insulators.

(ii) Employing simple “low temperature oxidation” to produce oxide filmsof highly controlled thickness, composition, purity and uniformity.

(iii) Avoiding inadvertent inter-diffusion of the metal and silicon aswell as silicide formation since the oxidation can be carried out atsuch low temperatures.

(iv) Using metal oxides that provide desirably lower tunnel barriers,relative to barriers currently used such as SiO₂.

(v) Providing a wide range of higher dielectric constant oxide filmswith improved capacitance characteristics.

(vi) Providing a unique ability to precisely tailor tunnel oxide barrierproperties for various device designs and applications.

(vii) Permitting the use of thicker tunnel barriers, if needed, toenhance device performance and its control along with yield andreliability.

(viii) Developing layered oxide tunnel barriers by atomic layerdeposition on multiple oxide layers in order, for example, to enhancedevice yields and reliability more typical of single insulating layers.

(ix) Eliminating soft erase errors caused by the current technique oftunnel erase from floating gate to the source.

FIG. 2 illustrates one embodiment of a floating gate transistor, ornon-volatile memory cell 200, according to the teachings of the presentinvention. As shown in FIG. 2, the non-volatile memory cell 200 includesa first source/drain region 201 and a second source/drain region 203separated by a channel region 205 in a substrate 206. A floating gate209 opposes the channel region 205 and is separated therefrom by a gateoxide 211. A control gate 213 opposes the floating gate 209. Accordingto the teachings of the present invention, the control gate 213 isseparated from the floating gate 209 by an asymmetrical low tunnelbarrier intergate insulator 215.

In one embodiment of the present invention, the asymmetrical low tunnelbarrier intergate insulator 215 includes an asymmetrical metal oxideinsulator which is aluminum oxide (Al₂O₃). In an alternative embodimentof the present invention, the asymmetrical low tunnel barrier intergateinsulator 215 includes an asymmetrical transition metal oxide selectedfrom the group consisting of Ta₂O₅, TiO₂, ZrO₂, and Nb₂O₅. In stillanother alternative embodiment of the present invention, theasymmetrical low tunnel barrier intergate insulator 215 includes anasymmetrical Perovskite oxide tunnel barrier selected from the groupconsisting of SrBi₂Ta₂O₃, SrTiO₃, PbTiO₃, and PbZrO₃

According to the teachings of the present invention, the floating gate209 includes a polysilicon floating gate 209 having a metal layer 216formed thereon in contact with the asymmetrical low tunnel barrierintergate insulator 215. Likewise, the control gate 213 includes apolysilicon control gate 213 having a metal layer 217, having a workfunction different from the metal layer 216 formed on the floating gate209, formed thereon in contact with the asymmetrical low tunnel barrierintergate insulator 215. In one embodiment, metal layer 216 is formed ofthe same metal material used to form the asymmetrical metal oxideinterpoly insulator 215. As stated above, the non-volatile memory cell,as described herein, can then be implemented in a number of programmablearray type logic and/or memory devices according to the teachings of thepresent invention.

FIG. 3 illustrates another embodiment of a floating gate transistor, ornon-volatile memory cell 300, according to the teachings of the presentinvention. As shown in the embodiment of FIG. 3, the non-volatile memorycell 300 includes a vertical non volatile memory cell 300. In thisembodiment, the non-volatile memory cell 300 has a first source/drainregion 301 formed on a substrate 306. A body region 307 including achannel region 305 is formed on the first source/drain region 301. Asecond source/drain region 303 is formed on the body region 307. Methodsfor forming such a vertical transistor structure are disclosed in U.S.Pat. No. 6,135,175, entitled “Memory Address Decode Array with verticaltransistors, which is incorporated herein by reference. A floating gate309 opposes the channel region 305 and is separated therefrom by a gateoxide 311. A control gate 313 opposes the floating gate 309. Accordingto the teachings of the present invention, the control gate 313 isseparated from the floating gate 309 by an asymmetrical low tunnelbarrier intergate insulator 315.

In one embodiment of the present invention, low tunnel barrier intergateinsulator 315 includes an asymmetrical metal oxide insulator which isaluminum oxide (Al₂O₃). In an alternative embodiment of the presentinvention, the asymmetrical low tunnel barrier intergate insulator 315includes an asymmetrical transition metal oxide selected from the groupconsisting of Ta₂O₅, TiO₂, ZrO₂, and Nb₂O₅. In still another alternativeembodiment of the present invention, the low tunnel barrier intergateinsulator 315 includes an asymmetrical Perovskite oxide tunnel barrierselected from the group consisting of SrBi₂Ta₂O₃, SrTiO₃, PbTiO₃, andPbZrO₃.

The floating gate 309 includes a polysilicon floating gate 309 having ametal layer 316 formed thereon in contact with the asymmetrical lowtunnel barrier intergate insulator 315. The control gate 313 includes apolysilicon control gate 313 having a metal layer 317, having a workfunction different from the metal layer 316 formed on the floating gate309, formed thereon in contact with the asymmetrical low tunnel barrierintergate insulator 315. As stated above, the non-volatile memory cell,as described herein, can then be implemented in a number of programmablearray type logic and/or memory devices according to the teachings of thepresent invention.

As shown in FIG. 3, the floating gate 309 includes a vertical floatinggate 309 formed alongside of the body region 307. In the embodimentshown in FIG. 3, the control gate 313 includes a vertical control gate313 formed alongside of the vertical floating gate 309.

As will be explained in more detail below, the floating gate 309 andcontrol gate 313 orientation shown in FIG. 3 is just one embodiment fora vertical non volatile memory cell 300, according to the teachings ofthe present invention. In other embodiments, explained below, thefloating gate includes a horizontally oriented floating gate formedalongside of the body region. In this alternative embodiment, thecontrol gate includes a horizontally oriented control gate formed abovethe horizontally oriented floating gate.

FIG. 4 is a perspective view illustrating an array of silicon pillars400-1, 400-2, 400-3, . . . , 400-N, formed on a substrate 406 as used inone embodiment according to the teachings of the present invention. Aswill be understood by one of ordinary skill in the art upon reading thisdisclosure, the substrates can be (i) conventional p-type bulk siliconor p-type epitaxial layers on p+ wafers, (ii) silicon on insulatorformed by conventional SIMOX, wafer bonding and etch back or silicon onsapphire, or (iii) small islands of silicon on insulator utilizingtechniques such as described in more detail in U.S. Pat. No. 5,691,230,by Leonard Forbes, entitled “Technique for Producing Small Islands ofSilicon on Insulator,” issued Nov. 25, 1997, which is incorporatedherein by reference.

As shown in FIG. 4, each pillar in the array of silicon pillars 400-1,400-2, 400-3, . . . , 400-N, includes a first source/drain region 401and a second source/drain region 403. The first and the secondsource/drain regions, 401 and 403, are separated by a body region 407including channel regions 405. As shown in FIG. 4, a number of trenches430 separate adjacent pillars in the array of silicon pillars 400-1,400-2, 400-3, . . . , 400-N. Trenches 430 are referenced in connectionwith the discussion which follows in connection with FIGS. 5A-5E.

FIGS. 5A-5E are cross sectional views taken along cut line 5-5 from FIG.4. As mentioned above in connection with FIG. 3, a number of floatinggate and control gate configurations are included in the presentinvention. FIG. 5A illustrates one such embodiment of the presentinvention. FIG. 5A illustrates a first source/drain region 501 andsecond source/drain region 503 for a non-volatile memory cell 500 formedaccording to the teachings of the present invention. As shown in FIG. 5,the first and second source/drain regions, 501 and 503, are contained ina pillar of semiconductor material, and separated by a body region 507including channel regions 505. As shown in the embodiments of FIGS.5A-5E, the first source/drain region 501 is integrally connected to aburied sourceline 525. As one or ordinary skill in the art willunderstand upon reading this disclosure the buried sourceline 525 is beformed of semiconductor material which has the same doping type as thefirst source/drain region 501. In one embodiment, the sourceline 525 isformed of semiconductor material of the same doping as the firstsource/drain region 501, but is more heavily doped than the firstsource/drain region 501.

As shown in the embodiment of FIG. 5A, a pair of floating gates 509-1and 509-2 are formed in each trench 530 between adjacent pillars whichform memory cells 500-1 and 500-2. Each one of the pair of floatinggates, 509-1 and 509-2, respectively opposes the body regions 507-1 and507-2 in adjacent pillars 500-1 and 500-2 on opposing sides of thetrench 530.

In this embodiment, a single control gate 513 is shared by the pair offloating gates 509-1 and 509-2 on opposing sides of the trench 530. Asone of ordinary skill in the art will understand upon reading thisdisclosure, the shared single control gate 513 can include an integrallyformed control gate line. As shown in FIG. 5A, such an integrally formedcontrol gate line 513 can be one of a plurality of control gate lineswhich are each independently formed in the trench, such as trench 530,below the top surface of the pillars 500-1 and 500-2 and between thepair of floating gates 509-1 and 509-2. In one embodiment, according tothe teachings of the present invention, each floating gate, e.g. 509-1and 509-2, includes a vertically oriented floating gate having avertical length of less than 100 nanometers.

As shown in the embodiment of FIG. 5B, a pair of floating gates 509-1and 509-2 are formed in each trench 530 between adjacent pillars whichform memory cells 500-1 and 500-2. Each one of the pair of floatinggates, 509-1 and 509-2, respectively opposes the body regions 507-1 and507-2 in adjacent pillars 500-1 and 500-2 on opposing sides of thetrench 530.

In the embodiment of FIG. 5B, a plurality of control gate lines areagain formed in trenches, e.g. trench 530, below the top surface of thepillars, 500-1 and 500-2, and between the pair of floating gates 509-1and 509-2. However, in this embodiment, each trench, e.g. 530, houses apair of control gate lines, shown as 513-1 and 513-2. Each one of thepair of control gate lines 513-1 and 513-2 addresses the floating gates,509-1 and 509-2 respectively, on opposing sides of the trench 530. Inthis embodiment, the pair of control gate lines, or control gates 513-1and 513-2 are separated by an insulator layer.

As shown in the embodiment of FIG. 5C, a pair of floating gates 509-1and 509-2 are again formed in each trench 530 between adjacent pillarswhich form memory cells 500-1 and 500-2. Each one of the pair offloating gates, 509-1 and 509-2, respectively opposes the body regions507-1 and 507-2 in adjacent pillars 500-1 and 500-2 on opposing sides ofthe trench 530.

In the embodiment of FIG. 5C, the plurality of control gate lines aredisposed vertically above the floating gates. That is, in oneembodiment, the control gate lines are located above the pair offloating gates 509-1 and 509-2 and not fully beneath the top surface ofthe pillars 500-1 and 500-2. In the embodiment of FIG. 5C, each pair offloating gates, e.g. 509-1 and 509-2, in a given trench shares a singlecontrol gate line, or control gate 513.

As shown in the embodiment of FIG. 5D, a pair of floating gates 509-1and 509-2 are formed in each trench 530 between adjacent pillars whichform memory cells 500-1 and 500-2. Each one of the pair of floatinggates, 509-1 and 509-2, respectively opposes the body regions 507-1 and507-2 in adjacent pillars 500-1 and 500-2 on opposing sides of thetrench 530.

In the embodiment of FIG. 5D, the plurality of control gate lines aredisposed vertically above the floating gates. That is, in oneembodiment, the control gate lines are located above the pair offloating gates 509-1 and 509-2 and not fully beneath the top surface ofthe pillars 500-1 and 500-2. However, in the embodiment of FIG. 5D, eachone of the pair of floating gates, e.g. 509-1 and 509-2, is addressed byan independent one of the plurality of control lines or control gates,shown in FIG. 5D as 513-1 and 513-2.

As shown in the embodiment of FIG. 5E, a single floating gate 509 isformed in each trench 530 between adjacent pillars which form memorycells 500-1 and 500-2. According to the teachings of the presentinvention, the single floating gate 509 can be either a verticallyoriented floating gate 509 or a horizontally oriented floating gate 509formed by conventional processing techniques, or can be a horizontallyoriented floating gate 509 formed by a replacement gate technique suchas described in a copending application, entitled “Flash Memory withUltrathin Vertical Body Transistors,” by Leonard Forbes and Kie Y. Ahn,application Ser. No. 09/780,169, now issued as U.S. Pat. No. 6,424,001.In one embodiment of the present invention, the floating gate 509 has avertical length facing the body region 505 of less than 100 nm. Inanother embodiment, the floating gate 509 has a vertical length facingthe body region 505 of less than 50 nm. In one embodiment, as shown inFIG. 5E, the floating gate 509 is shared, respectively, with the bodyregions 507-1 and 507-2, including channel regions 505-1 and 505-2, inadjacent pillars 500-1 and 500-2 located on opposing sides of the trench530. In one embodiment, the control gate 513 includes a horizontallyoriented control gate 513 formed above the horizontally orientedfloating gate 509.

As one of ordinary skill in the art will understand upon reading thisdisclosure, in each of the embodiments described above in connectionwith FIGS. 5A-5E the floating gates 509 are separated from the controlgate lines, or control gates 513 with an asymmetrical low tunnel barrierintergate insulator in accordance with the descriptions given above inconnection with FIG. 3. The modifications here are to use tunnelingthrough the interpoly dielectric to realize flash memory devices. Thevertical devices include an extra flexibility in that the capacitors,e.g. gate oxide and intergate insulator, are easily fabricated withdifferent areas. This readily allows the use of very high dielectricconstant inter-poly dielectric insulators with lower tunneling barriers.

FIGS. 6A-6D illustrate that a number of address coincidence schemes canbe used together with the present invention. FIG. 6A illustrates a NORflash memory array 610 having a number of non-volatile memory cells600-1, 600-2, 600-3, using a coincidence address array scheme. Forpurposes of illustration, FIG. 6A shows a sourceline 625 coupled to afirst source/drain region 601 in each of the number of non-volatilememory cells 600-1, 600-2, 600-3. The sourceline is shown oriented in afirst selected direction in the flash memory array 610. In FIG. 6A, anumber of control gate lines 630 are shown oriented in a second selecteddirection in the flash memory array 610. As shown in FIG. 6A, the numberof control gate lines 630 are coupled to, or integrally formed with thecontrol gates 613 for the number of non-volatile memory cells 600-1,600-2, 600-3. As shown in FIG. 6A, the second selected direction isorthogonal to the first selected direction. Finally, FIG. 6A shows anumber of bitlines 635 oriented in a third selected direction in theflash memory array 610. As shown in FIG. 6A, the number of bitlines arecoupled to the second source/drain regions in the number of non-volatilememory cells 600-1, 600-2, 600-3. In the embodiment shown in FIG. 6A thethird selected direction is parallel to the second selected directionand the number of control gate lines 630 serve as address lines. Also,as shown in FIG. 6A, the flash memory array 610 includes a number ofbackgate or substrate/well bias address lines 640 coupled to thesubstrate.

Using FIG. 6A as a reference point, FIGS. 6B-6D illustrate of top viewfor three different coincidence address scheme layouts suitable for usewith the present invention. First, FIG. 6B provides the top view layoutof the coincidence address scheme described in connection with FIG. 6A.That is, FIG. 6B illustrates a number of sourcelines 625 oriented in afirst selected direction, a number of control gate lines 630 oriented ina second selected direction, and a number of bitlines 635 oriented in athird selected direction for the flash memory array 600. As explainedabove in connection with FIG. 6A, in this embodiment, the second andthird selected direction are parallel to one another and orthogonal tothe first selected direction such that the number of control gate lines630 serve as address lines.

FIG. 6C provides the top view layout of another coincidence addressscheme according to the teachings of the present invention. This is,FIG. 6C illustrates a number of sourcelines 625 oriented in a firstselected direction, a number of control gate lines 630 oriented in asecond selected direction, and a number of bitlines 635 oriented in athird selected direction for the flash memory array 600. In theembodiment of FIG. 6C, the first selected direction and the thirdselected direction are parallel to one another and orthogonal to thesecond selected direction. In this embodiment, the number of controlgate lines 630 again serve as address lines.

FIG. 6D provides the top view layout of yet another coincidence addressscheme according to the teachings of the present invention. This is,FIG. 6D illustrates a number of sourcelines 625 oriented in a firstselected direction, a number of control gate lines 630 oriented in asecond selected direction, and a number of bitlines 635 oriented in athird selected direction for the flash memory array 600. In theembodiment of FIG. 6D, the first selected direction and the secondselected direction are parallel to one another and orthogonal to thethird selected direction. In this embodiment, the number of bitlines 635serve as address lines.

As will be apparent to one of ordinary skill in the art upon readingthis disclosure, and as will be described in more detail below, writecan still be achieved by hot electron injection and/or, according to theteachings of the present invention, tunneling from the control gate tothe floating gate. According to the teachings of the present invention,block erase is accomplished by driving the control gates with arelatively large positive voltage and tunneling from the metal on top ofthe floating gate to the metal on the bottom of the control gate.

FIG. 7A is an energy band diagram illustrating the band structure atvacuum level with the asymmetrical low tunnel barrier interpolyinsulator according to the teachings of the present invention. FIG. 7Ais useful in illustrating the reduced tunnel barrier off of the floatinggate to the control gate and for illustrating the respectivecapacitances of the structure according to the teachings of the presentinvention.

FIG. 7A shows the band structure of the silicon substrate, e.g. channelregion 701, silicon dioxide gate insulator, e.g. gate oxide 703,polysilicon floating gate 705, the asymmetrical low tunnel barrierinterpoly dielectric 707, between metal plates 709 and 711, and then thepolysilicon control gate 713, according to the teachings of the presentinvention.

The design considerations involved are determined by the dielectricconstant, thickness and tunneling barrier height of the asymmetricalinterpoly dielectric insulator 707 relative to that of the silicondioxide gate insulator, e.g. gate oxide 703. The tunneling probabilitythrough the interpoly dielectric 707 is an exponential function of boththe barrier height and the electric field across this dielectric.

FIG. 7A shows the asymmetrical tunnel barriers, formed by atomic layerdeposition (ALD), used for easy erase. Erase is achieved by the use ofpositive control gate voltages through the low tunnel barrier. In oneembodiment, according to the teachings of the present invention, readutilizes positive control gate voltages with n-channel enhancement modedevices as described in the above referenced, copending applications, bythe same inventors, entitled “FLASH MEMORY DEVICES WITH METAL OXIDEAND/OR LOW TUNNEL BARRIER INTERPLOY INSULATORS,” attorney docket number1303.014US1, application Ser. No. 09/945,507, now issued as U.S. Pat.No. 7,068,544; “PROGRAMMABLE MEMORY ADDRESS AND DECODE DEVICES WITHMETAL OXIDE AND/OR LOW TUNNEL BARRIER INTERPLOY INSULATORS,” attorneydocket number 1303.029US1, application Ser. No. 09/945,500, now issuedas U.S. Pat. No. 7,075,829; “FIELD PROGRAMMABLE LOGIC ARRAYS WITH METALOXIDE AND/OR LOW TUNNEL BARRIER INTERPLOY INSULATORS, attorney docketnumber 1303.027US1, application Ser. No. 09/945,512, now issued as U.S.Pat. No. 7,087,954; “DEAPROM WITH INSULATING METAL OXIDE INTERPLOYINSULATORS,” attorney docket number 1303.024US1, application Ser. No.09/945,498, now issued as U.S. Pat. No. 6,778,441. In anotherembodiment, according to the teachings of the present invention, readutilizes negative control gate voltages with n-channel depletion modedevices as described in the above referenced, copending application, bythe same inventors, entitled “PROGRAMMABLE ARRAY TYPE LOGIC AND/ORMEMORY DEVICES WITH METAL OXIDE AND/OR LOW ASYMMETRICAL TUNNEL BARRIERINTERPLOY INSULATORS,” attorney docket number 1303.020US1, applicationSer. No. 09/943,134, now issued as U.S. Pat. No. 7,042,043. In anotherembodiment, according to the teachings of the present invention, readutilizes negative control gate voltages with p-channel enhancement modedevices as described in the above referenced, copending application, bythe same inventors, entitled “PROGRAMMABLE ARRAY TYPE LOGIC OR MEMORYWITH P-CHANNEL DEVICES AND ASYMMETRICAL TUNNEL BARRIERS,” attorneydocket number 1303.035US1, application Ser. No. 10/028,001, now issuedas U.S. Pat. No. 7,132,71 1. Programming is accomplished by channel hotelectron injection with n-channel devices and/or electron injection fromthe control gate for both n-channel and p-channel devices and may or maynot utilize positive substrate, well, or body bias.

FIG. 7B is an energy band diagram illustrating the band structure duringan erase operation of electrons from the floating gate 705 to thecontrol gate 713 across the low tunnel barrier interpoly insulator 707according to the teachings of the present invention. FIG. 7B issimilarly useful in illustrating the reduced tunnel barrier off of thefloating gate 705 to the control gate 713 and for illustrating therespective capacitances of the structure according to the teachings ofthe present invention.

As shown in FIG. 7B, the electric field is determined by the totalvoltage difference across the structure, the ratio of the capacitances(see FIG. 7A), and the thickness of the asymmetrical interpolydielectric 707. The voltage across the asymmetrical interpoly dielectric707 will be, ΔV2=V C1/(C1+C2), where V is the total applied voltage. Thecapacitances, C, of the structures depends on the dielectric constant,∈_(r), the permittivity of free space, ∈_(o), and the thickness of theinsulating layers, t, and area, A, such that C=∈_(r)∈_(o)A/t,Farads/cm², where ∈_(r) is the low frequency dielectric constant. Theelectric field across the asymmetrical interpoly dielectric insulator707, having capacitance, C2, will then be E2=ΔV2/t2, where t2 is thethickness of this layer.

The tunneling current in erasing charge from the floating gate 705 bytunneling to the control gate 713 will then be as shown in FIG. 7B givenby an equation of the form:J=B exp(−Eo/E)where E is the electric field across the interpoly dielectric insulator707 and Eo depends on the barrier height. Practical values of currentdensities for aluminum oxide which has a current density of 1 A/cm² at afield of about E=1V/20 Å=5×10⁺⁶ V/cm are evidenced in a description byPollack. (See generally, S. R. Pollack and C. E. Morris, “Tunnelingthrough gaseous oxidized films of Al₂O₃,” Trans. AIME, Vol. 233, p. 497,1965). Practical current densities for silicon oxide transistor gateinsulators which has a current density of 1 A/cm² at a field of aboutE=2.3V/23A=1×10⁺⁷ V/cm are evidenced in a description by T. P. Ma et al.(See generally, T. P. Ma et al., “Tunneling leakage current in ultrathin(<4 nm) nitride/oxide stack dielectrics,” IEEE Electron Device Letters,vol. 19, no. 10, pp. 388-390, 1998).

The lower electric field in the aluminum oxide interpoly insulator 707for the same current density reflects the lower tunneling barrier ofapproximately 2 eV, shown in FIG. 7B, as opposed to the 3.2 eV tunnelingbarrier of silicon oxide 703, also illustrated in FIG. 7B.

FIG. 7C is a graph plotting tunneling currents versus the appliedelectric fields (reciprocal applied electric field shown) for a numberof barrier heights. FIG. 7C illustrates the dependence of the tunnelingcurrents on electric field (reciprocal applied electric field) andbarrier height. The fraction of voltage across the asymmetricalinterpoly or asymmetrical intergate insulator, ΔV2, can be increased bymaking the area of the intergate capacitor, C2, (e.g. intergateinsulator 707) smaller than the area of the transistor gate capacitor,C1 (e.g. gate oxide 703). This would be required with high dielectricconstant intergate dielectric insulators 707 and is easily realized withthe vertical floating gate structures described above in connection withFIGS. 3, and 5A-5E.

Methods of Formation

As described above, this disclosure describes the use of asymmetricaltunnel barriers formed by atomic layer deposition (ALD) and specificallylow tunnel barriers during erase, to make erase of flash memory typedevices easier. In conventional flash memory type devices with tunnelerase from the floating gate to the transistor source, the silicon oxidepresents a very high 3.2 eV barrier and high electric fields arerequired. The combination of very high electric fields and damage by hotelectron collisions in the oxide result in a number of operationalproblems like soft erase error, reliability problems of premature oxidebreakdown and a limited number of cycles of write and erase. Thetunneling currents depend exponentially on the barrier heights. Anasymmetrical barrier, as shown in FIG. 7A, presents a low barrier forerase but can present a higher barrier during read and/or writeoperations when tunneling is not desired. These asymmetrical barrierscan be achieved various ways, one technique is to use different metalcontact plates, with the upper plate on the interpoly or intergateinsulator being a metal like platinum with a large work function asdescribed above in the referenced, copending application, by the sameinventors, entitled “PROGRAMMABLE ARRAY LOGIC OR MEMORY DEVICES WITHASYMMETRICAL TUNNEL BARRIERS,” attorney docket number 1303.020US1,application Ser. No. 09/943,134, now issued as U.S. Pat. No. 7,042,043.

Key characteristics of ultra-thin ALD oxides for this invention includethe following:

-   -   (a) Films can be grown at low (<300 to 400 degrees Celsius)        temperatures.    -   (b) Films can be grown on a variety of substrate materials,        including a wide range of inorganic (e.g., silicon, glass, oxide        and nitride) to metallic surfaces.    -   (c) Films can be comprised of single (e.g., Al₂O₃) and multiple        metal components.    -   (d) The thicknesses of the oxide films can be controlled to        within a thickness of 1 monolayer. Their thickness uniformities        are exceptionally high.    -   (e) Films are chemically homogeneous, uniform and have a strong        tendency to form the most stable compositions in their        respective metal-oxygen systems. For example, Ta₂O₅, forms in        the Ta—O system.    -   (f) Even ultra-thin films exhibit excellent step and sidewall        coverage. This will be particularly advantageous for enhancing        the quality of so-called “vertical transistor” devices. Step        coverage difficulties are relatively less demanding in        “horizontally configured” transistors.    -   (g) Control of the bottom metal layer thickness and uniformity        are less demanding, provided the metal is sufficiently        conductive throughout. In other words, the prime function of the        metal, in combination with the appropriate oxide, is to produce        a lower tunnel barrier relative to conventional barriers such as        Si/SiO₂.    -   (h) Films are excellent insulators with high breakdown        strengths.    -   (i) Films have high dielectric constants as formed at low        temperatures. This invention does not require that the oxides        have very high dielectric constants. However, if necessary, many        of the ALD oxides can be subsequently heat treated to        substantially increase their dielectric constants. Such changes        typically result from minute micro-structural changes, i.e.,        transformations from amorphous to nano-crystalline phases.

As is well-known to those in the field, the literature describing ALDoxide processes is quite large and still expanding rapidly. Withinreasonable constraints imposed by chemical and physical properties ofcomponent metals and their oxides, ALD processes can be developed forproducing an even wider range of single and multi-component oxide thinfilms. A few examples of ALD processes for forming some useful oxidesfor tunnel barriers and other applications are given next.

Al₂O₃ Films. A variety of ALD processes have been described for makinghigh quality, ultra-thin Al₂O₃ films. Thus Kim et al. (see generally, Y.K. Kim et al., “Novel capacitor technology for high density, stand-aloneand embedded DRAMs”, IEDM (2000)) describe the use of TMA and ozone toform superior Al₂O₃ films on silicon at 350C for DRAM applications. J.H. Lee et al. (see generally, J. H. Lee, et al., “Effect of polysilicongate on the flatband voltage shift and mobility degradation forALD-Al₂O₃ gate dielectric”, IEDM (2000); D-G Park et al.,“Characteristics of n⁺ polycrystalline-Si/Al₂O₃/Simetal-oxide-semiconductor structures prepared by atomic layer chemicalvapor deposition using Al(Ch₃)₃ and H₂O vapor”, Jour. Appl. Phys. 89(11), pp. 6275-6280 (2001)) from the same laboratory investigated theeffects of doped poly-silicon gate electrode layers on the properties ofAl₂O₃ films formed by ALD at 450 degrees Celsius and crystallized at 850degrees Celsius and found that interfacial dopant segregation canimprove capacitive characteristics. In quite a different application,Paranjpe et al. (see generally, A. Paranjpe et al., “Atomic layerdeposition of AlO_(x), for thin film head gap applications”, Jour.Electrochem. Soc. 148 (9), G465-G471 (2001)) developed an ALD process(using TMA to form Al precursor layers and oxidizing them with water) toproduce excellent AlO_(x), layers at 150-200 degrees Celsius for use inadvanced gap and tunnel junction devices. Alumina films, grown onsubstrates as diverse as Si, Ta and NiFe were amorphous, conformal,stoichiometric (to within 2 at. %, pure (<5 at. % hydrogen and <1 at. %other impurities), smooth (R_(A)˜2 angstroms) with controllable levelsof stress. Extraordinary levels of thickness control (to within 1angstrom) have been achieved upon using ALD to form ultra-thin Al₂O₃ andSiO₂ films on BN particles. Such oxide coatings can be employed tomodify surface energies in order to increase loading of BN particles inpolymer films for packaging. (See generally, J. D. Ferguson et al.,“Atomic layer deposition of Al₂O₃ and SiO₂ on BN particles usingsequential surface reactions”, Appl. Surf. Sci. 162-163, pp. 280-292(2000)).

Transition Metal (TM) Oxide Films. Various ALD processes have also beendescribed for producing ultra-thin Ta₂O₅ films. Kim et al. (seegenerally, Y. S. Kim et al., “Effect of rapid thermal annealing on thestructure and the electrical properties of atomic layer deposited Ta₂O₅films”, Jour. Korean Phys. Soc. 37 (6), pp. 975-979 (2000)) have grownsuch oxide films on Si wafers and ITO glasses at 300 degrees Celsius byreacting Ta(OEt)₅ and water. Films made under 550 degrees Celsius wereamorphous. Their dielectric constant increased with RTA temperature.With increasing RTA temperatures, leakage initially became smaller andthen increased as crystallization became more evident. Such effects areknown to depend upon RTA ambients and Ta₂O₅ underlayers and must beoptimized according to applications to produce useful tunnel barriers.

Smith et al. (see generally, R. C. Smith et al., “Chemical vapourdeposition of the oxides of titanium, zirconium and hafnium for use ashigh-k materials in microelectronic devices. A carbon-free precursor forthe synthesis of hafnium dioxide”, Advanced Materials for Optics andElectronics, 10, pp. 105-114 (2001)) surveyed Group IV metal precursorsused for the formation of thin TiO₂, ZrO₂ and HfO₂ films by both CVD andALCVD processes in an effort to produce oxides that are completelycarbon-, hydrogen- and halogen-free. They found, for example, thatHf(NO₃)₄ could be used for making CVD HfO₂ films on silicon attemperatures as low as 300 degrees Celsius. Such films contain excessoxygen and that can apparently be removed by heating in nitrogen athigher temperatures. Similar findings were made forming TiO₂ and ZrO₂films. The authors opine that these oxides could be made by ALCVD, usingGroup IV nitrate precursors. Due to the low ALD operable temperatures,it is possible to conduct detailed in situ deposition rate studies inmany instances in order to more precisely define metal oxidethicknesses. For example, Aarik et al. (see generally, J. Aarik et al,“Anomalous effect of temperature on atomic layer deposition of titaniumdioxide”, Jour. Crystal Growth 220, pp. 531-537 (2000). See also K.Kukli et al., “Real time monitoring in atomic layer deposition of TiO₂from TiI₄ and H₂O−H₂O₂”, Langmuir 16, pp. 8122-8128 (2000)) used aTiCl₄+H₂O ALD process to grow TiO₂ films on quartz QCM substrates:deposition rate, refractive index and related properties were highlydependent on deposition temperature in the 150 to 225 degrees Celsiusrange. This unexpected high dependency resulted from unusual surfaceroughening due to the simultaneous formation of amorphous andcrystalline TiO₂ phase at the higher temperatures. Other crystallineTiO₂ phases can co-deposit at the ALD temperatures are raised to 300 to400 degrees Celsius. Arrik et al. (see generally, J. Aarik et al.,“Texture development in nanocrystalline hafnium dioxide thin films grownby atomic layer deposition”, Jour. Crystal Growth 220, pp. 105-113(2000)) have also investigated a broader range of temperature effects onHfO₂ formed on SiO₂ and Si by ALD. Using HfCl₄ and H₂O as precursors,they found that amorphous films were formed at 225 degrees Celsius butcrystalline films were formed at 300 degrees Celsius and above. Oxidefilms grown for microelectronic applications should desirably beamorphous in order to avoid porosity and grain boundaries causing highleakage currents (see generally, K. Kukli et al., “Atomic layerdeposition of zirconium oxide from zirconium tetraiodide, water andhydrogen peroxide”, Jour. Crystal Growth 231, pp. 262-272 (2001)).Zirconium dioxide films have attractively high dielectric constants butgenerally low breakdown values due, presumably, to their strong tendencyto crystallize. The results of Kukli et al. suggest that ALDtemperatures under 250 degrees Celsius should yield the desiredamorphous structure when using a ZrI₄ precursor.

It has been shown that mixtures of transition metal oxides can also bedeposited for use as tunnel barriers. Such processes involve depositingtransition metal alloy precursor layers followed by oxidation, followedagain by addition of the alloy precursor layer and so on until thedesired mixed oxide tunnel junction thickness is reached. Likewise ithas been shown that certain perovskite oxide oxide films can be formedby first using ALD to form the desired amorphous oxide composition andthen heating to produce the perovskite crystal structure. Clearlyformation of such mixed oxide films is more complex and will not bedescribed here.

Process Descriptions

Two examples, according to the present invention, are sketched out belowfor building asymmetrical Metal/ALD Oxide/Metal tunnel barriers overpoly-Si floating gate electrodes. Additional background and fabricationdetails are in earlier disclosures, as referenced herein, dependent onthe particular ALD systems employed and is otherwise available to thoseskilled in the art. If patterning and other processes do not imposeconstraints, one could use an ALD system that is modified by theaddition of a second processing chamber for depositing in situ thebottom and top metal layers. This multichamber system would provideimproved control over key oxide tunnel barrier properties, especiallythickness, composition and interfacial impurities.

EXAMPLE I

Formation of Al/Al₂O₃/Al tunnel barriers can be built having a barrierheight of about ˜2 eV. FIG. 8 graphically illustrates the dependence ofthe barrier height for current injection on the work function andelectron affinity of a given, homogeneous dielectric film. FIG. 9 is atable which provides relevant data on the barrier heights, energy gaps,dielectric constants and electron affinities of a wide variety of metaloxides that could be used as asymmetric tunnel barriers according to theteachings of the present invention. (See generally, H. F. Luan et al.,“High quality Ta2O5 gate dielectrics with Tox equil. 10 Angstroms,” IEDMTech. Digest, pp. 141-144, 1999; J. Robertson et al., “Schottky barrierheights of tantalum oxide, barium strontium titanate, lead titanate andstrontium bismuth tantalate,” App. Phys. Lett., Vol. 74, No. 8, pp.1168-1170, February 1999; J. Robertson, “Band offsets of wide-band-gapoxides and implications for future electronic devices,” J. Vac. Sci.Technol. B, Vol. 18, No. 3, pp. 1785-1791, 2000; Xin Guo et al., “Highquality ultra-thin (1.5 nm) TiO2/Si3N4 gate dielectric for deepsubmicron CMOS technology,” IEDM Tech. Digest, pp. 137-140, 1999; H.-S.Kim et al., “Leakage current and electrical breakdown in metal-organicchemical vapor deposited TiO2 dielectrics on silicon substrates,” Appl.Phys. Lett., Vol. 69, No. 25, pp. 3860-3862, 1996; J. Yan et al.,“Structure and electrical characterization of TiO2 grown from titaniumtetrakis-isoproxide (TTIP) and TTIP/H2O ambient,” J. Vac. Sci. Technol.B, Vol. 14, No. 3, pp. 1706-1711, 1966).

Neglecting patterning steps along the way, the processing sequence canbe:

-   -   (i) Use a low energy, inert ion plasma in the auxiliary chamber        to sputter clean residual oxides, etc. from the poly-Si surfaces        previously fabricated on the device wafer.    -   (ii) Deposit an aluminum contact layer over the poly-Si. This        layer is presumably ten to hundreds of angstroms thick, as        needed to insure good coverage of the poly-Si.    -   (iii) Transfer the device wafer to the ALD processing chamber        under a vacuum sufficient to prevent inadvertent oxidation.    -   (iv) Produce the desired Al₂O₃ layer via an ALD. Several        precursor chemistries are available, as indicated by the few        examples in the references cited above in connection with the        discussion on Al₂O₃. For example, the low temperature process        described by Paranjpe et al. (see generally, A. Paranjpe et al.,        “Atomic layer deposition of AlO_(x), for thin film head gap        applications”, Jour. Electrochem. Soc. 148 (9), G465-G471        (2001)) looks attractive for this purpose since it was developed        to operate at temperatures in the 150-200 degrees Celsius range,        using trimethylaluminum and water as precursors. As such, Al₂O₃        films as thin as 5 to 10 Angstroms have been shown to be        continuous with excellent insulating properties.    -   (v) Transfer the device wafer back to the auxiliary chamber and        deposit the top aluminum electrode layer.    -   (vi) Remove the wafer from the system for further processing,        e.g., addition of silicon control layer, patterning, etc.

Barriers of Al₂O₃ formed on Al will exhibit some minor barrier heightdifference when injecting electrons from the inner and outer electrodes.The barrier height difference will be at most 0.1 eV and will arise fromsmall differences in oxide composition at the interfaces. (See generallycopending application, entitled “PROGRAMMABLE ARRAY TYPE LOGIC OR MEMORYWITH P-CHANNEL DEVICES AND ASYMMETRICAL TUNNEL BARRIERS,” attorneydocket number 1303.035US1, application Ser. No. 10/028,001, now issuedas U.S. Pat. No. 7,132,711, for a complete explanation). Moreover, suchsmall differences will not interfere with the proper functioning of thedevices of this disclosure.

EXAMPLE II

Formation of Al/Ta₂O₅/Al tunnel barriers can be formed with a barrierheight of about 2 eV. See FIGS. 8 and 9. Again, processes for producingultra-thin films of Ta₂O₅ that are suitable for tunnel barriers areknown. See, for example, the work of Kim et al. (see generally, Y. S.Kim et al., “Effect of rapid thermal annealing on the structure and theelectrical properties of atomic layer deposited Ta₂O₅ films”, Jour.Korean Phys. Soc. 37 (6), pp. 975-979 (2000)) cited above. Note that itmay not be necessary to maximize the dielectric constant of this oxidefor the present applications although such maximization is desirable forbuilding useful, minimal area DRAM storage capacitors. One can fabricatethese Al/Ta₂O₅/Al tunnel barriers following the approach sketched outabove.

-   -   (i) Use a low energy, inert ion plasma in the auxiliary chamber        to sputter clean residual exodies, etc. from the poly-Si        surfaces previously fabricated on the device wafer.    -   (ii) Deposit an aluminum contact layer over the poly-Si. This        layer is presumably ten to hundreds of angstroms thick, as        needed to insure good coverage of the poly-Si.    -   (iii) Transfer the device wafer to the ALD processing chamber,        under a vacuum sufficient to prevent inadvertent oxidation.    -   (iv) Produce the desired Ta₂O₅ layer via an ALD process such as        the one just cited, using a Ta(OEt)₅ and water as precursors and        a temperature of 300 degrees Celsius or lower, if possible, in        order to prevent inadvertent Al recrystallization and growth.        Formed in this way, the dielectric constant of the oxide will be        approximately 22-24.    -   (v) Transfer the device wafer back to the auxiliary chamber and        deposit the top aluminum electrode layer.    -   (vi) Remove the wafer from the system for further processing,        e.g., addition of silicon control layer, patterning, etc.

A very limited intermixing of Al and Ta oxides at the ALD formedinterface can develop unless a few steps are taken to minimize this. Forexample, minimization of the ALD process temperature. Alternatively,first forming a monolayer of Al₂O₃ by exposing the water precursorbefore the Ta(OEt)₅ precursor. Intermixing of a monolayer or two at thisinterface can also be accepted (provided it is reproducible fromwafer-to-wafer, run-to-run, etc.). More detailed studies have shown thatthe tunnel current-barrier thickness characteristics are betterdescribed in terms of an “average barrier height.” Clearly the largebulk, if not all, of the tunnel barrier will consist of a layer of Ta₂O₅with a thickness that could lie in the range of perhaps 20 to 50Angstroms or more. Upon reflection of a variety of metal/oxide tunnelbarriers, it is evident that “nature abhors perfect interfaces.” Even inthe Si/SiO₂ system which is perhaps the one that approaches most nearlyto perfection.

System Level

FIG. 10 illustrates a block diagram of an embodiment of an electronicsystem 1001 according to the teachings of the present invention. In theembodiment shown in FIG. 10, the system 1001 includes a memory device1000 which has an array of memory cells 1002, address decoder 1004, rowaccess circuitry 1006, column access circuitry 1008, control circuitry1010, and input/output circuit 1012. Also, as shown in FIG. 10, thecircuit 1001 includes a processor 1014, or memory controller for memoryaccessing. The memory device 1000 receives control signals from theprocessor 1014, such as WE*, RAS* and CAS* signals over wiring ormetallization lines. The memory device 1000 is used to store data whichis accessed via I/O lines. It will be appreciated by those skilled inthe art that additional circuitry and control signals can be provided,and that the memory device 1000 has been simplified to help focus on theinvention. At least one of the processor 1014 or memory device 1000 hasa memory cell formed according to the embodiments of the presentinvention. That is, at least one of the processor 1014 or memory device1000 includes an asymmetrical low tunnel barrier interpoly insulatoraccording to the teachings of the present invention.

It will be understood that the embodiment shown in FIG. 10 illustratesan embodiment for electronic system circuitry in which the novel memorycells of the present invention are used. The illustration of system1001, as shown in FIG. 10, is intended to provide a generalunderstanding of one application for the structure and circuitry of thepresent invention, and is not intended to serve as a completedescription of all the elements and features of an electronic systemusing the novel memory cell structures. Further, the invention isequally applicable to any size and type of memory device 1000 using thenovel memory cells of the present invention and is not intended to belimited to that described above. As one of ordinary skill in the artwill understand, such an electronic system can be fabricated insingle-package processing units, or even on a single semiconductor chip,in order to reduce the communication time between the processor and thememory device.

Applications containing the novel memory cell of the present inventionas described in this disclosure include electronic systems for use inmemory modules, device drivers, power modules, communication modems,processor modules, and application-specific modules, and may includemultilayer, multichip modules. Such circuitry can further be asubcomponent of a variety of electronic systems, such as a clock, atelevision, a cell phone, a personal computer, an automobile, anindustrial control system, an aircraft, and others.

Conclusions

Asymmetrical low barrier tunnel insulators are described between thefloating gate and control gate in a flash memory type devices to formprogrammable array logic and memory devices. The asymmetrical lowbarrier insulators, ˜2.0 eV, are formed by atomic layer deposition(ALD). While the amount of charge stored on the floating gate is smallthe transistor provides gain and charge multiplication resulting in alarge output signal and ease of reading the stored data. If there is anadverse capacitance ratio due to a large difference of dielectricconstants then the vertical gate structures described previously can beemployed.

It has been shown that the asymmetrical low tunnel barrier interpolyinsulators of the present invention avoid the large barriers to electrontunneling or hot electron injection presented by the siliconoxide-silicon interface, 3.2 eV, which result in slow write and erasespeeds even at very high electric fields. The present invention alsoavoids the combination of very high electric fields and damage by hotelectron collisions in the which oxide result in a number of operationalproblems like soft erase error, reliability problems of premature oxidebreakdown and a limited number of cycles of write and erase. Further,the asymmetrical low tunnel barrier interploy dielectric insulator eraseapproach, of the present invention remedies the above mentioned problemsof having a rough top surface on the polysilicon floating gate whichresults in, poor quality interpoly oxides, sharp points, localized highelectric fields, premature breakdown and reliability problems.

The use of ALD greatly increases the capability of forming a given metaloxide insulator on a dissimilar metal contact layer. This abilityprovides a much increased latitude in independently selecting chemicallyand physically superior contact metals and ALD metal oxidescombinations. Judicious selection of the contact metal/ALD oxide couplealso provides flexibility in setting the electron tunneling barrierheight over ranges not possible via the thermal oxidation approach. Thisdissimilar contact metal may also function as a diffusion barrier. Thismay be required when high temperature treatments are used subsequentlyto increase the dielectric constant of the oxide.

The above mentioned problems with programmable array type logic and/ormemory devices and other problems are addressed by the present inventionand will be understood by reading and studying the followingspecification. Systems and methods are provided for programmable arraytype logic and/or memory devices with asymmetrical, low tunnel barrierinterpoly insulators.

In one embodiment of the present invention, a non-volatile memory cell,or floating gate transistor, includes a first source/drain region and asecond source/drain region separated by a channel region in a substrate.A floating gate opposes the channel region and is separated therefrom bya gate oxide. A control gate opposes the floating gate. The control gateis separated from the floating gate by an asymmetrical low tunnelbarrier intergate insulator. The low tunnel barrier intergate insulatorincludes a metal oxide insulator selected from the group consisting ofAl₂O₃, Ta₂O₅, TiO₂, ZrO₂, Nb₂O₅, SrBi₂Ta₂O₃, SrTiO₃, PbTiO₃, and PbZrO₃.The floating gate includes a polysilicon floating gate having a metallayer formed thereon in contact with the low tunnel barrier intergateinsulator. And, the control gate includes a polysilicon control gatehaving a metal layer, having a different work function from the metallayer formed on the floating gate, formed thereon in contact with thelow tunnel barrier intergate insulator.

These and other embodiments, aspects, advantages, and features of thepresent invention will be set forth in part in the description, and inpart will become apparent to those skilled in the art by reference tothe following description of the invention and referenced drawings or bypractice of the invention. The aspects, advantages, and features of theinvention are realized and attained by means of the instrumentalities,procedures, and combinations particularly pointed out in the appendedclaims.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. It is to be understood that the above description is intendedto be illustrative, and not restrictive. Combinations of the aboveembodiments, and other embodiments will be apparent to those of skill inthe art upon reviewing the above description. The scope of the inventionincludes any other applications in which the above structures andfabrication methods are used. The scope of the invention should bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

1. A nonvolatile memory, comprising: a number of pillars extendingoutwardly from a substrate, wherein each pillar includes a firstsource/drain region, a body region, and a second source/drain region; anumber of floating gates opposing the body regions in the number ofpillars; an oxide intermediate at least one of the floating gates and atleast one of the body regions, respectively; a number of control gatesopposing the floating gates; a number of sourcelines disposed below thenumber of pillars and coupled to the first source/drain regions along afirst direction in the array of memory cells; a number of control gatelines formed integrally with the number of control gates along a seconddirection in the array of flash memory cells, wherein the number ofcontrol gates are separated from the floating gates by a low tunnelbarrier intergate insulator having a tunneling barrier of less than 2.0eV and having a number of small compositional ranges such that gradientscan be formed by an applied electric field to produce different barrierheights at an interface with the floating gate and the control gate; anda number of bitlines coupled to the second source/drain regions along athird direction in the array of flash cells.
 2. The nonvolatile memoryof claim 1, wherein the low tunnel barrier intergate insulator includesa metal oxide insulator that includes lead.
 3. The nonvolatile memory ofclaim 1, wherein the low tunnel barrier intergate insulator includes ametal oxide insulator that includes strontium.
 4. The nonvolatile memoryof claim 1, wherein the low tunnel barrier intergate insulator includesa metal oxide insulator that includes titanium.
 5. The nonvolatilememory of claim 1, wherein at least one floating gate includes apolysilicon layer and a metal layer with the metal layer being over thepolysilicon layer and in contact with the asymmetrical low tunnelbarrier intergate insulator.
 6. The nonvolatile memory of claim 5,wherein at least one control gate includes a polysilicon control gatehaving a metal layer formed thereon in contact with the low tunnelbarrier intergate insulator, wherein the metal layer formed on thepolysilicon control gate includes a metal layer that has a differentwork function than the metal layer formed on the floating gate.
 7. Thenonvolatile memory of claim 5, wherein the metal layer includes a parentmetal for the asymmetrical low tunnel barrier intergate insulator andthe metal layer formed on the control gate includes a metal layer havinga work function in the range of 2.7 eV to 5.8 eV.
 8. The nonvolatilememory of claim 1, wherein at least one floating gate is a verticalfloating gate formed in a trench below a top surface of each pillar suchthat each trench houses a pair of floating gates opposing the bodyregions in adjacent pillars on opposing sides of the trench.
 9. Thenonvolatile memory of claim 6, wherein a plurality of control gate linesis formed in the trench below the top surface of the pillar and betweenthe pair of floating gates, wherein each pair of floating gates shares asingle control gate line, and wherein each floating gate includes avertically oriented floating gate having a vertical length of less than100 nanometers.
 10. The nonvolatile memory of claim 6, wherein aplurality of control gate lines is formed in the trench below the topsurface of the pillar and between the pair of floating gates such thateach trench houses a pair of control gate lines each addressing thefloating gates one on opposing sides of the trench respectively, andwherein the pair of control gate lines are separated by an insulatorlayer.
 11. The nonvolatile memory of claim 6, wherein a plurality ofcontrol gate lines is disposed vertically above the floating gates, andwherein each pair of floating gates shares a single control gate line.12. The nonvolatile memory of claim 6, wherein a plurality of controlgate lines is disposed vertically above the floating gates, and whereineach one of the pair of floating gates is addressed by an independentone of the plurality of control gate lines.
 13. The nonvolatile memoryof claim 1, wherein each floating gate is a horizontally orientedfloating gate formed in a trench below a top surface of each pillar suchthat each trench houses a floating gate opposing the body regions inadjacent pillars on opposing sides of the trench, and wherein eachhorizontally oriented floating gate has a vertical length of less than100 nanometers opposing the body region of the pillars.
 14. Thenonvolatile memory of claim 13, wherein a plurality of control gatelines is disposed vertically above the floating gates.
 15. Thenonvolatile memory of claim 1, wherein the array is incorporated into amemory device that is coupled to a processor to form an electronicsystem.
 16. The nonvolatile memory of claim 1, wherein the sourceline isformed of material that has the same doping type as a first source/drainregion to which the buried sourceline is coupled, the buried sourcelinebeing more heavily doped than the first source/drain region.
 17. Thenonvolatile memory of claim 8, wherein a pair of floating gates share asingle control gate disposed between the pair of floating gates.
 18. Thenonvolatile memory of claim 8, wherein the pair of floating gates sharea single control gate disposed vertically above the pair of floatinggates.
 19. The nonvolatile memory of claim 1, wherein a single floatinggate is disposed in a trench below a top surface of a pair of adjacentpillars, the floating gate being shared by body regions of the adjacentpillars.
 20. The nonvolatile memory of claim 19, wherein a control gateis disposed vertically above the single floating gate.
 21. Aprogrammable logic array, comprising: a plurality of input lines forreceiving an input signal; a plurality of output lines; and one or morearrays having a first logic plane and a second logic plane connectedbetween the input lines and the output lines, wherein the first logicplane and the second logic plane comprise a plurality of logic cellsarranged in rows and columns for providing a sum-of-products term on theoutput lines responsive to a received input signal, wherein theplurality of logic cells includes a logic cell comprising: a firstsource/drain region formed on a substrate; a body region including achannel region formed on the first source/drain region; a secondsource/drain region formed on the body region; a floating gate opposingthe channel region and separated therefrom by a gate oxide; a controlgate opposing the floating gate; and wherein the control gate isseparated from the floating gate by a low tunnel barrier intergateinsulator having a tunneling barrier of less than 2.0 eV, and having anumber of small compositional ranges such that gradients can be formedby an applied electric field which produce different barrier heights atan interface with the floating gate and control gate.
 22. Theprogrammable logic array of claim 21, wherein the asymmetrical lowtunnel barrier intergate insulator includes a metal oxide insulatorselected from the group consisting of Al₂O₃, Ta₂O₅, TiO₂, ZrO₂, Nb₂O₅,SrBi₂Ta₂O₃, SrTiO₃, PbTiO₃, and PbZrO₃.
 23. The programmable logic arrayof claim 21, wherein the floating gate includes a polysilicon floatinggate having a metal layer formed thereon in contact with theasymmetrical low tunnel barrier intergate insulator.
 24. Theprogrammable logic array of claim 21, wherein the control gate includesa polysilicon control gate having a metal layer formed thereon incontact with the asymmetrical low tunnel barrier intergate insulator,wherein the metal layer formed on the polysilicon control gate includesa metal layer that has a different work function than the metal layerformed on the floating gate.
 25. The programmable logic array of claim21, wherein metal layer formed on the floating gate includes a parentmetal for the asymmetrical low tunnel barrier intergate insulator andthe metal layer formed on the control gate includes a metal layer havinga work function in the range of 2.7 eV to 5.8 eV.
 26. An electronicsystem, comprising: a processor; and a memory device coupled to theprocessor, wherein at least one of the memory device and the processorincludes an array of flash memory cells, comprising: a number of pillarsextending outwardly from a substrate, wherein each pillar includes afirst source/drain region, a body region, and a second source/drainregion; a number of floating gates opposing the body regions in thenumber of pillars and separated therefrom by a gate oxide; a number ofcontrol gates opposing the floating gates; a number of buriedsourcelines disposed below the number of pillars and coupled to thefirst source/drain regions along a first selected direction in the arrayof memory cells; a number of control gate lines formed integrally withthe number of control gates along a second selected direction in thearray of flash memory cells, wherein the number of control gates areseparated from the floating gates by a low tunnel barrier intergateinsulator having a tunneling barrier of less than 2.0 eV, and having anumber of small compositional ranges such that gradients can be formedby an applied electric field which produce different barrier heights atan interface with the floating gate and control gate; and a number ofbitlines coupled to the second source/drain regions along a thirdselected direction in the array of flash cells.
 27. The electronicsystem of claim 26, wherein the asymmetrical low tunnel barrierintergate insulator includes a metal oxide insulator selected from thegroup consisting of Al₂O₃, Ta₂O₅, TiO₂, ZrO₂, Nb₂O₅, SrBi₂Ta₂O₃, SrTiO₃,PbTiO₃, and PbZrO₃.
 28. The electronic system of claim 26, wherein thefloating gate includes a polysilicon floating gate having a metal layerformed thereon in contact with the low tunnel barrier intergateinsulator.
 29. The electronic system of claim 26, wherein the controlgate includes a polysilicon control gate having a metal layer formedthereon in contact with the low tunnel barrier intergate insulator,wherein the metal layer formed on the polysilicon control gate includesa metal layer that has a different work function than the metal layerformed on the floating gate.
 30. The electronic system of claim 29,wherein metal layer formed on the floating gate includes a parent metalfor the asymmetrical low tunnel barrier intergate insulator and themetal layer formed on the control gate includes a metal layer having awork function in the range of 2.7 eV to 5.8 eV.
 31. The electronicsystem of claim 26, wherein the floating gate is a vertical floatinggate formed in a trench below a top surface of each pillar such thateach trench houses a pair of floating gates opposing the body regions inadjacent pillars on opposing sides of the trench.
 32. The electronicsystem of claim 31, wherein the plurality of control gate lines areformed in the trench below the top surface of the pillar and between thepair of floating gates, wherein the pair of floating gates shares asingle control gate line, and wherein the floating gate includes avertically oriented floating gate having a vertical length of less than100 nanometers.
 33. The electronic system of claim 31, wherein theplurality of control gate lines are formed in the trench below the topsurface of the pillar and between the pair of floating gates such thatthe trench houses a pair of control gate lines each addressing thefloating gates one on opposing sides of the trench respectively, andwherein the pair of control gate lines are separated by an insulatorlayer.
 34. The electronic system of claim 26, wherein the plurality ofcontrol gate lines are disposed vertically above the floating gates, andwherein each pair of floating gates shares a single control gate line.35. The electronic system of claim 26, wherein the plurality of controlgate lines are disposed vertically above the floating gates, and whereineach of the pair of floating gates is addressed by an independent one ofthe plurality of control lines.
 36. The electronic system of claim 26,wherein the floating gate is a horizontally oriented floating gateformed in a trench below a top surface of each pillar such that eachtrench houses a floating gate opposing the body regions in adjacentpillars on opposing sides of the trench, and wherein each horizontallyoriented floating gate has a vertical length of less than 100 nanometersopposing the body region of the pillars.
 37. The electronic system ofclaim 26, wherein the plurality of control gate lines are disposedvertically above the floating gates.